Add release guidelines (opendistro-for-elasticsearch#38)

* Add release guidelines

Author: VijayanB

docs/guide/release.md

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+# Release guidelines
+
+
+odfe-cli versions are expressed as **x.y.z**, where **x** is the major version, **y** is the minor version, and **z** is the patch version, following [Semantic Versioning](https://semver.org/) terminology. 
+
+**The major, minor & patch version is related to odfe-cli compatibility, instead of odfe’s compatibility.**
+
+
+**Q: What goes into new patch version release train?**
+
+* Known bugs that prevent users from executing tasks. 
+    * eg: Failed to delete non-running detectors by delete command, failed to handle error.
+* Security vulnerabilities. 
+    * eg: leaking user credentials, insecure interaction with components.
+* cli’s dependency is updated with new patch version which fixes bugs and security issues that affect your feature. (Treat your dependent library as your library)
+    * eg: cobra released a new version which fixes some security vulnerabilities like [#1259](https://github.com/spf13/cobra/pull/1259)
+
+Any committer, who fixed issues, **related to above use case**, should notify admin to initiate new patch release. There is no SLA for patch releases, since this is triggered based on severity of the issue.
+
+
+
+**Q: What goes into new minor version release train?**
+
+* Any new commands ( could be a new plugin, a new sub command for any plugin ). A new command could be a new API that was released long ago but being included in new odfe-cli release or new API that will be  released in next odfe release.
+    * eg: on-board [k-nn](https://github.com/opendistro-for-elasticsearch/odfe-cli/pull/20/) plugin, add auto-complete feature for cli commands.
+* New parameter/flags for any command.
+    * eg: odfe-cli 1.0.0 only displays detector configuration using get command for given name, if user would also like to see [detector job](https://opendistro.github.io/for-elasticsearch-docs/docs/ad/api/#get-detector), they can add new flag (job) to get command to enable this feature.
+* Any incompatible changes that were introduced with respect to API changes.
+    * eg: if API is added in odfe 1.13.0 and updated in a backward incompatible way in later releases, this will be addressed in CLI as minor version
+
+admin will trigger new minor release train whenever new plugin is on-boarded or every 45 days if any commits related to above use case is added. After every minor release, admin will create a new table where contributors can include candidates for next release.
+
+
+
+**Q**: **What goes into new major version release train?**
+
+* Any changes related to framework or addition of new component with respect to odfe-cli.
+    * eg: Use new rest library (networking layer), framework re-design to ease new plugin on-boarding, user experience changes like update command formats, rename command names, etc. 
+
+admin will trigger new major release train every 120 days if any commits related to above use case is added. After every major release, admin will create a new table related to next release where contributors can include candidates for next release.
+
+